Last week I reinstalled my Yanmar 2GM and was quite careful with the realignment -certainly within .002" at the flanges. Now when powering forward I get a thumping sound at 2200 RPM or above; no sound when in reverse. When crawling around listening while underway, the noise seems to come from the hull. The cutlass bearing seemed fine when I put the boat in this spring. Where should I start?

Sounds like the shaft "whipping" and hitting the shaft log or worn mounts sagging moving to much under power and allowing the shaft to make contact with the shaft log.. Do you have a two blade prop, folding prop?? Before you started the alignment the shaft was dead center in the shaft log, correct..??

Thank you all for your helpful responses. I have a two-bladed Martec prop, which is in good condition. I had the prop rebalanced as part of recent routine maintenance.

Since I pulled the engine in the water and have a face seal rather than a stuffing box, I thoroughly braced the shaft in position before pulling the engine so that the shaft would move forward. I didn't check to see that the shaft was dead center in the center of the log. I will do so when i check the mounts later today

In addition to the foregoing suggestions, you might want to take a look at the shaft strut. We had a similar issue that proved to be the shaft strut moving up-and down as the prop rotated. It doesn't take much movement to generate quite a loud thump.

"It is not so much for its beauty that the sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."

OP, you say that the "thumping" happens at 2200 RPM or above. Does it get worse as you increase the RPM, stay the same or diminish? Are you sure that the prop is clean? Is it new? If so, are you sure it is weighted and balanced right? If a new prop, correct pitch? Do you have zincs between the prop shaft log and the strut or after the strut between the propeller and the strut? Are they new or are they worn? Could they be loose? If you put the transmission into neutral and rev the engine, do you feel the vibration? If it was the motor mounts, I would think you would feel it somewhat.

While the shaft is turning, when looking at the turning in the stuffing box into the shaft log is it bouncing around a lot?

Answers to these will shed more light on the problem. I don't necessarily think it is a bad mount.

Thank you all for your helpful responses. I have a two-bladed Martec prop, which is in good condition. I had the prop rebalanced as part of recent routine maintenance.

Since I pulled the engine in the water and have a face seal rather than a stuffing box, I thoroughly braced the shaft in position before pulling the engine so that the shaft would move forward. I didn't check to see that the shaft was dead center in the center of the log. I will do so when i check the mounts later today

These props are notorious for causing shaft whip. I have owned them and know it all too well. If you have a longish shaft between the cutlass bearing and the gear box output flange these two blade props can literally turn the shaft into a jump rope between the two bearings. If your shaft was not 100% centered in the shaft log this "whip" can cause the shaft to thump the shaft log at certain RPM.

Folding or many feathering props move the blade center considerably further aft of the cutlass bearing. This coupled with the fact that it is a two blade prop, non geared etc. can lead to vibration and shaft whip issues.

I am personally currently dealing with this on our own boat with a Flex-O-Fold two blade prop. See; FOLDING PROP REPORT The calculations put her on the borderline for whip, and I took the chance... Missed.... The shaft is just flat out too long for a folding two blade and it causes MASSIVE vibrations at certain RPM. This despite the shaft being true to .001" and the alignment also to .001" and the prop having been run through a Prop Scan tool to confirm balance.. The prop is coming off this week and the three blade will go back on..

Blades further aft cause more vibration:

Here's the off set on a fixed prop and an exaggerated illustration of what shaft whip looks like.

Here's a photo of a customers boat that was experiencing massive shaft whip thumping. We were able to get rid of the thumping and get everything to .001" including alignment and shaft tolerance.

Thank you all for your suggestions. After work yesterday my visual inspection the engine mounts and vigorously rocking the engine by hand found no problems with motor mounts. In hopes of determining whether the prop shaft is centered I disconnected the two flanges, pushed the prop shaft aft from the output flange on the transmission, and wiggled the prop shaft side to side and up and down. The face seal precluded a definitive determination of whether the prop shaft is on dead center of the stern tube but it looked pretty good. So I decided to look at it from the outside. After rebolting the flanges together, I motored in and put the boat in slings to inspect the prop, zincs, shaft and how centered the shaft is where it enters the stern tube. Based upon an eye ball inspection everything looked okay except the shaft was about 1/64" off center on the horizontal plane. Rotating the shaft by hand didn't reveal any bend in the shaft. I ran out of time so I put it back in the water and motored back to the mooring with the engine cover off so I could inspect the engine mounts under load. I ran it up to 2,600 RPM's and the mounts appeared to be fine and there was NO THUMP!

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